#include <functional> // std::is_placeholder, std::placeholders
#include <iostream>   // std::cout, std::boolalpha

int main() {
  using namespace std::placeholders; // introduces _1

  std::cout << std::is_placeholder<decltype(_1)>::value << '\n'; // 1
  std::cout << std::is_placeholder<decltype(_2)>::value << '\n'; // 2
  std::cout << std::is_placeholder<int>::value << '\n';          // 0

  return 0;
}
